Mission Calls By Provider Endpoint

Provider breakdown of calls in the window.

Uses the SAME UTC-day window as /mission/calls/daily so the per-provider totals sum to the per-day totals exactly. Previously this used a rolling N*24h window (now - N days), which bled slightly into the hour-old edge of the oldest day and excluded a few hours of the current day — causing the by-provider sum to diverge from the calls chart by a few thousand rows on busy weeks.